Electron beam furance with material-evaporant equilibrium control

ABSTRACT

A vacuum coating device having an evaporation chamber and a rod fed electron beam gun, wherein the end (target) of a consumable rod is melted and vaporized by the electron beam; the meniscus of the melt being kept by automatic control at a constant height and in equilibrium with the evaporation rate of the rod material; the control comprising an electric servo motor for advancing the rod at a controlled rate into the evaporation chamber, a photocell subject to radiation from the rod melt, a fixed mask with a graduated aperture for varying the sensed radiation according to varying height of the meniscus, and a control amplifier responsive to the photocell for supplying a rate control voltage to the servo motor.

FY5511? XR United States Patent n91 Norwalk [75] Inventor: Marshall II. Norwalk, Williamsville,

[73] Assignee: Airco, Inc., New York, NY,

[22] Filed; Aug. 24, 1971 [21] App]. No.: 174,385

[52] US. Cl. ..l3/3l, 250/201, 318/640 [51] Int. Cl. ..H05b 7/00 [58] Field ofSearch ..13/1,31,34;

[56] References Cited UNITED STATES PATENTS 451 May 1, 1973 3.300.618 1/1967 Sciaky ..l3/31 X Primary Examiner-Roy N. Envall, Jr. AllomeyLarry R. Cassett et a1.

[5 7] ABSTRACT A vacuum coating device having an evaporation chamber and a rod fed electron beam gun, wherein the end (target) of a consumable rod is melted and vaporized by the electron beam; the meniscus of the melt being kept by automatic control at a constant height and in equilibrium with the evaporation rate of the rod material; the control comprising an electric servo motor for advancing the rod at a controlled rate into the evaporation chamber, a photocell subject to radiation from the rod melt, a fixed mask with a graduated aperture for varying the sensed radiation according to varying height of the meniscus, and a control amplifier responsive to the photocell for supplying a rate control voltage to the servo motor.

13 Claims, 2 Drawing Figures 3,106,594 10/1963 Beasley et a1 ..13/1 3,274,417 9/1966 Haefer et all ..l3/31 X 1 24 E T. 1 l

(1 AMP 2 Patented May 1, 1973 INVENTOR. MARSHALL H. NORWALK 8 W0 ATTORNEY ELECTRON BEAM FURANCE WITH MATERIAL- EVAPORANT EQUILIBRIUM CONTROL BACKGROUND OF THE INVENTION The field of the invention encompasses electron beam vapor deposition furnaces of the type having an evacuated evaporation chamber and a crucible within which a material composed of one or more constituents is vaporized by an electron beam. The evaporation chamber may also contain a workpiece such as sheet material, herein referred to as a substrate, that is to be coated by the evaporant.

It is known that the properties ofa film deposited on a substrate by the evaporation of a material in vacuum depend to a great extent on the rate at which the material is evaporated and on the composition of the evaporant.

Where a multi-constituent material is involved and the individual constituents have different rates of evaporation under any given conditions, difficulty has been encountered in evaporating a constant known composition at a constant known rate. This is because the individual components must be replenished in such a way that the rate of replenishment of each component is in equilibrium with the rate ofits evaporation, and the method of replenishment can cause a significant change in the conditions affecting evaporation.

For example, it is especially important during longterm evaporation that the melt does not steadily recede into the crucible since excessive recession will cause extreme instability in the molten pool and therefore in the rate of evaporation and composition of the evaporant. It is of equal importance that the meniscus does not steadily rise during the evaporation since excessive increase in height will affect the energy input into the material, drastically changing the normal balance of the system, or causing the molten material to spill onto the top of the crucible, or both. Accordingly, either non-equilibrium position of the meniscus tends to adversely affect rate of evaporation and the composition of the evaporant.

Heretofore, there has been no satisfactory control for readily achieving uniform rate of evaporation with uniform composition of the evaporant under equilibrium conditions in electron beam furnaces.

SUMMARY OF THE INVENTION In accordance with the invention, a material composed of one or more constituents to be evaporated in vacuum is automatically fed into an electron beam at a rate for insuring, after establishment of equilibrium conditions, that the composition of the evaporant is constant and unvarying, and that the constituents of the evaporant over long-term evaporation are in the same proportion as existed in the rod material being fed into the molten pool in the crucible. In particular, the material, preferably in rod form, is fed into an evaporation chamber wherein the top or target end of the rod is melted by the beam from an electron gun. A constant evaporation rate is established by technique known in the art, and the rod is mechanically fed by a servo control system into the electron beam at a rate such that the height of the surface or meniscus of the molten pool or melt at the top of the rod is constant relative to a fixed reference point in the chamber.

Subject of course to keeping the evaporation rate and environment reasonably constant with time, feeding of the rod into the molten pool at a rate equivalent to that at which material is evaporated from the pool, insures uniformity between the composition of the evaporant and the composition of the rod. This uniformity remains as long as there are no significant changes in any of the evaporation variables.

The servo rod-feed control system for maintaining at constant height the meniscus of the rod metal preferably comprises a servo motor that is mechani cally connected to the rod for advancing it into the evaporation chamber in accordance with a motor rate control voltage that is derived by sensing means according to variation in the sensed height or level of the meniscus relative to the fixed reference point, representing the condition of uniformity described above.

With the control system of the invention, an electron beam source is capable of depositing on a substrate a film or coating of a predictably uniform and constant composition throughout comparatively long periods of evaporation.

A principal object of the invention is an improved control system for feeding vaporizable material into the evaporation chamber of an electron beam furnace at a rate insuring equilibrium between the replenishment of the material being evaporated and the evaporation of material thereby maintaining uniformity between the composition of the evaporant and the composition of the vaporizable material.

A further object is an improved control system of the character described above wherein the level of the material melted by the electron beam within the evaporation chamber is automatically maintained substantially constant relative to a fixed reference level during long-term evaporation of the material.

A related object is an improved servo control system wherein the meniscus level of the material melt is sensed by photosensitive means for controlling the speed of a servo mechanism that in turn feeds the material into the evaporation chamber at a controlled rate.

A further object is an improved fast-response control system of the character described above that is readily adjustable for maintaining equilibrium between the rate of replenishment of the vaporizing material and the selected rate of evaporation by the electron beam, and that is simple and inexpensive.

Other objects, features and advantages will appear from the following description with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I. is a schematic illustration of an optical-electromechanical feed control system for material to be evaporated in an electron beam furnace, embodying the present invention; and

FIG. 2 is a partial front view of an optical mask used in the sensing system of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT The system details and apparatus of the invention illustrated by FIGS. 1 and 2 will first be described with this to be followed by a description of a preferred mode of practicing the invention, together with a consideration of the significant factors that in general are involved in the operation of present electron beam furnaces and are related to the use of the invention.

The electron beam evaporation system generally represented in FIG. 1 comprises an evacuated chamber EC containing a conventional electron beam gun G, a crucible C for containing the target material M that is subject to melting and evaporation by the deflected electron beam B, and the substrate S to be coated with a film of the evaporant. The substrate, according to operation requirements, may for example take the form of a plate as indicated, or a continuous sheet; in the latter case the evaporation chamber is provided with conventional vacuum locks for passing the sheet through the chamber for continuous coating by the evaporant. The evaporation chamber, where preferred, may also contain elements of the material feed control system, such as portions of the optics, sensing and servo drive mechanisms that are described below.

In accordance with a preferred form of the invention, the crucible C is provided with a vertical bore terminating at its upper end in a small crucible cavity 12. The material M to be vaporized is in the form of a rod 14 that is guided through the bore 10 to cavity 12 and the heating electron beam B. The target end 16 of the rod which is melted by the heating beam ordinarily terminatcs at or within the cavity, whereas the rod melt l8 fills the cavity so that its meniscus at extends above the upper surface of the crucible. In order to maintain the meniscus 20 at the desired level, the rate of replenishment of the melt 18 equals the rate of material evaporation. The rod 14 is advanced into the evaporation chamber and crucible by a servo mechanism generally indicated at SM. The rate of advance is governed by an optical sensing system responsive to the height of the meniscus.

Referring first to the electro-optical sensing system foi controlling the servo mechanism SM, the molten material in the crucible at 18 emits radiation in the visible spectrum that is focused upon a photosensitive device such as a photocell 22. The internal resistance of the photocell indicated at 24, varies inversely with the intensity of the received radiation for controlling the motor circuitry as described below. Radiation from the melt is directed to and focused on the photocell 22 as diagrammatically indicated, through an optical system comprising a fixed radiation screen or mask 26, transparent moving film 28, evaporation chamber window 30, optical filter 32 and focusing lens 34.

The mask 26, H6. 2, has at its upper end an aperture ofgraduated cross-sectional area, such as for example a V-shaped notch 36 that is horizontally aligned with the melt so that the desired or reference level of the meniscus 20 (when projected horizontally) appears as a line 20' within the notch 36. Accordingly, the intensity of radiation from the melt (assuming its level to be at the reference line 20') is directly related to the crosssectional area within the notch represented by the shaded area. Where the meniscus is above the level of reference line 20, as at 20a for example, the radiation area seen through the notch ,by the focusing lens 34 is materially increased and a proportionately greater amount of radiation passes through the mask aperture. Conversely, lowering of the meniscus to a level as at 20b, results in greatly reduced passage of radiation through the mask.

lt follows, therefore, that when the meniscus height rises above the reference 20 the increased radiation focused on the sensing device 22 causes the resistance 24 to decrease and the resistance will increase when the meniscus falls below the reference.

Since it is important that the intensity of radiation reaching the photocell 22 remain constant for a given level of the melt meniscus over a period of long term evaporation, the moving transparent film 28 is interposed between the evaporating melt and the evaporation chamber window 30 for insuring uniform density of the radiation path. The film is moved in front of the chamber window at a constant rate by a simple tape drive device 38 including a small constant speed motor M. Progressive accumulation of evaporant from the chamber tending to coat the window 30 and gradually reduce the intensity of transmitted radiation is thereby precluded. As fresh film is continuously moving in front of the window, the deposition of evaporant thereon can be considered negligible.

The resistance 24 ofthe photosensitive device is connected in conventional circuitry for controlling the servo mechanism SM so that the speed of the servo mechanism corresponds to the magnitude of the resistance. That is, the servo increases its speed for advancing the material rod 14 at a faster rate into the crucible when the value of resistance 24 increases above its reference value (corresponding to radiation intensity at the desired level 20' of the meniscus), and decreases its speed when the resistance drops below its reference value, as for example when the meniscus has increased to a height represented by line 200, FIG. 2.

To this end, the sensing device 22 is connected by lines 40 to an amplifier 42 having a linear characteristic. The amplifier output, which is therefore dependent on the signal developed from the photocell, is connected by lines 44 to a conventional servo motor 46 having a linear voltage-speed characteristic. The motor output is suitably connected through mechanical transmission and reduction gearing at 48 and 50 respectively, to a lead screw 52 that in turn is provided with a traveling nut 54 having a lateral rod support at 56 for advancing the rod into the evaporation chamber. The opposite balancing end of the rod support is kept in proper alignment by suitable means such as a guide rod 58.

As the output of the control amplifier 42 is the determining factor in the rate of rod feed, a volt meter 60 and recorder 62 can be connected across the line 4 as indicated for routine monitoring of system control.

Summarizing, the ideal steady state condition of the optical electro-mechanical control system of the invention is represented by a constant feed operation of the servo motor controlled by a steady photocell signal (determined by the radiation sensitive resistance 24) that in turn corresponds to a constant reference height of the melt meniscus, i.e. a steady state condition wherein equilibrium is established between the rate of rod feed into the melt pool and the rate of evaporation therefrom.

SYSTEM OPERATION As mentioned above, an equilibrium condition must be established as a basis for uniform evaporation rate control. A prerequisite to obtaining this condition is stability of the molten pool itself. Any instability in the pool which changes the evaporation rate independent of external control must be eliminated before the control state described above can be achieved. For example, purity of the evaporating material is required as the surface of the pool must be free of oxides or other foreign material, since such materials can act as a variable barrier to evaporation. Thermal equilibrium must be reached so that the area of the evaporating surface remains constant and no material spills from the pool 'onto the shoulder around the crucible; also, the pool must not boil such that its shape is distorted.

When evaporating a multi-component material, such as an alloy mixture or composite, sufficient evaporation time must be allowed to insure that a composition gradient at equilibrium with the evaporation conditions has been established. That is, an equilibrium composition gradient must be established in the feed rod to assure that no one constituent of a multicomponent material will be fed at a rate which will either increase or decrease the percentage of that material in the evaporant relative to the original composition of the feed material. For example, if a rod of approximate composition 50% Ni, 50% Cr is evaporated, the initial evaporation rate is very high relative to that experienced during equilibrium evaporation, and the major constituent of the evaporant is chromium. As evaporation continues under constant environmental conditions, the evaporation rate decreases gradually and the percentage of chromium in the evaporant also decreases. This process continues until the equilibrium evaporation rate is reached, at which time the composition of the evaporant is the same as the composition of the feed rod.

However, at this time, the composition of the molten pool (due to the difference in the vapor pressure of nickel and chromium) is much higher in nickel than the feed rod, and the composition of the solid portion of the feed rod in the area heated by thermal conduction from the molten pool varies between (i) the composition of the pool (at the liquid-solid interface) and (ii) the composition of the rod at a point where the temperaturc-pressurc condition is no longer suitable for sublimation ofthe chromium.

Equilibrium conditions having been established, this composition gradient will be maintained in the solid portion ofthe rod immediately below the molten pool as the rod is fed into the pool. A major change in any condition of evaporation, such as power input, cooling rate, chamber pressure, etc., requires that a new composition gradient be established before the system can again be considered to be at equilibrium. As mentioned above, time must be allowed for this composition adjustment to be completed.

Assuming that normal operating conditions obtain, it is possible in practicing the invention to maintain controllable conditions even though the chamber pressure may vary cyclicly within limits, as might happen for example when moving substrates into or from the evaporation chamber by conventional vacuum lock techniques.

The start up of an electron beam and the stabilization evaporation rate control stabilized at the equilibrium condition is accomplished as follows. With no power on the system, the top end of the feed rod 14 is adjusted for location slightly below the position (reference) desired at equilibrium. The electron beam gun G is energized from the high voltage line RV and the beam of electrons B that is focused by magnetic deflection on the top of the feed rod, heats the rod, causing it to melt and subsequently to evaporate. The tape drive at 38 is turned on causing the optically transparent tape 28 to pass slowly in front of the chamber window 30 and optical system. The control amplifier 42 and the circuit including the photocell 22 is energized for supplying power to the rod drive motor 46.

As described above, the optical system is arranged so that light radiation from the molten pool 18 passes through the V-mask 26, the transparent tape 28, the optical filter 32 (where used), and is focused by the lens 34 (or lens system) on the plane of the photo-sensitive element of the photocell 22 for producing the control signal. While focus is not critical, it should be such that a recognizable image of the melt pool can exist at the photocell. The photocell can be connected in any number of conventional and well known circuits such that a change in sensed light radiation from the molten pool 18 causes the voltage across the photocell to vary inversely relative to increase or decrease in intensity of the radiation. The photocell voltage accordingly causes the output of the control amplifier 42 to vary correspondingly so that the feed rod 14 is driven more slowly when the melt meniscus (as at 20a) is above the desired control height 20' and faster when the meniscus (as at 20b) is below the desired control height. That is, when the meniscus is above the desired control height, the area of radiating pool surface, and therefore the amount of light seen by the photocell is greater than when the pool meniscus is at the desired equilibrium height. Therefore, the resistance of the photocell is comparatively low, with correspondingly low voltage across the photocell and the speed of the rod drive motor (in turn, directly responsive to the now lower control-amplifier output) is lower than required for maintaining the meniscus at the increased height 20a. Accordingly, the meniscus drops slowly and as it does the amount of light radiated to the photocell gradually decreases, thereby retarding the deceleration rate of the motor. Equilibrium control is achieved when the meniscus has stabilized at a point recognized as the reference height 20' (HO. 2), where the light radiated from the meniscus regulates the rod drive motor (ideally at a constant speed) as required to feed the rod into the molten pool at the same rate the material is evaporated from the pool.

If the meniscus control height established by this technique should be too low for good evaporation conditions to exist, the reference control height may be raised by introducing optical filters into the optical path as at 32, to reduce the amount of light focused on the photocell. This results in higher photocell resistance and an increase of rod drive speed for raising the height of the meniscus. For a readily variable optical filter, crossed polars are particularly useful for this purpose.

From the description of the correction control above, it will be apparent that when the meniscus falls below the desired control height, as at b of FIG. 2, the area of the radiating pool surface, and therefore the amount of light seen by the photocell, will be less than when the meniscus is at the control height 20. The rod drive motor will then receive higher voltage and its speed, therefore,--will be higher than is required to maintain the meniscus at the control height. The meniscus rises slowly and as it does the amount oflight available to the photocell increases, causing the motor gradually to slow down as the meniscus again approaches the control height. As in the previous case, control is achieved when the rate of rod-feed and the rate of evaporation are balanced.

if the meniscus control height established by the technique above should be too high for good evaporation conditions to exist, a photocell of greater compensatory sensitivity, i.e. higher light to dark ratio, may be used.

It should also be noted that the V-mask serves two significant purposes in the system: one, it selects a specified portion of the meniscus for control, and two, it increases the sensitivity of the system by causing a large change in light passed for a small change in the height of the meniscus. it should be understood that the shape of the mask aperture is not limited to the V- notch used by way of example. It may vary in geometric configuration for relating sensed radiation intensity to meniscus height as preferred.

The degree of stability of the output of the control amplifier 42 is a direct measure of the degree of control achieved, and is directly proportional, once a stable equilibrium has been achieved, to the evaporation rate. Thus, the accuracy of control can be continuously checked by reading the amplifier output on the volt meter 60 and/or recorder 62. Therefore, having determined the relationship between voltage and evaporation rate for a given material under specific conditions of evaporation, this voltage can be used as a guide for determining evaporation rates.

Although the evaporation chamber as disclosed, contains but part of the optical and rod drive system, all

' elements of the control system except the amplifier and volt meter-recorder can, where required, be located within the chamber. Alternatively, all elements of the control system except the transparent film and its drive for shielding the chamber window from deposited evaporant, can be located outside the chamber, generally in the manner illustrated.

Having set forth the invention in what is considered to be the best embodiment thereof, it will be understood that changes may be made in the system and apparatus as above set forth without departing from the spirit of the invention or exceeding the scope thereof as defined in the following claims.

I claim:

1. In an electron beam evaporation system having an evaporation chamber, an electron beam gun and a consumable target composed of material to be evaporated from a melt within the chamber, the method of maintaining an established equilibrium evaporation rate of the material which comprises:

a. sensing the intensity of light radiation from the material at the meniscus level of the melt;

b. varying the intensity of the sensed radiation in accordance with changes in the meniscus level thereof; and

c. supplying the material to the melt at a rate according to the intensity of the sensed radiation for maintaining the meniscus level at the constant equilibrium level.

2. The method as specified in claim 1 wherein the melt at the meniscus is a source of light radiation, and the radiated light for sensing is masked and restricted to a path of graduated cross-sectional area which varies in size with variations in the meniscus level.

3. In an electron beam evaporation system having an evaporation chamber, an electron beam gun for generating an electron beam, a consumable target for the beam composed of material to be melted to form a melt and then evaporated within the chamber, and drive means for replenishing the vaporized target material so as to maintain a constant equilibrium evaporation rate, the improvement comprising a. sensing means responsive to radiation from the target melt and for producing signal corresponding to the magnitude of such radiation,

b. means for varying the intensity of radiation received at the sensing means according to the height of the melt meniscus,

c. drive control means connected to the said signal and responsive thereto to control the rate of replenishing of the vaporized material at a rate for maintaining the meniscus at the constant level.

4. An electron beam evaporation system as specified in claim 3 wherein the material is in the form of a rod and the advancing end of the rod constitutes the target for the electron beam.

5. An electron beam evaporation system as specified in claim 3 wherein the sensing means includes photosensitive means for producing a control signal for the drive means.

6. An electron beam evaporation system as specified in claim 5 wherein the drive means comprises an electric servo mechanism that is responsive to the control signal for advancing the rod toward the electron beam.

7. An electron beam evaporation system as specified in claim 3 wherein the means for varying the intensity of radiation comprises a fixed mask having an aperture of graduated cross-sectional area interposed between the melt meniscus and the sensing means, whereby the intensity of sensed radiation varies according to the meniscus height.

8. An electron beam evaporation system as specified in claim 7 wherein the mask aperture consists of a V- shaped notch, the notch at its median width being in general horizontal alignment with the melt meniscus at the constant reference height.

9. An electron beam evaporation system as specified in claim 3 including an expendable transparent evaporant shield disposed within the chamber in the path of the variable intensity radiation to the sensing means, and shield moving means connected thereto for continuously moving the evaporant exposed area thereof from the radiation path.

10. An electron beam evaporation system as specified in claim 5 further including an optical system between said melt and said sensing means having means for restricting radiation and focusing the restricted radiation on the photosensitive means.

11. An electron beam evaporation system as specified in claim 4 wherein the evaporation chamber includes a crucible having a vertical bore, and the rod is advanced by the drive means through the bore and into the crucible.

12, An electron beam evaporation system as specified in claim 11 wherein the level of the melt meniscus extends beyond the upper surface of the crucible for passing radiation horizontally to the sensing means.

13. An electron beam evaporation system having an evaporation chamber containing an electron beam gun, a crucible for vertically guiding into the electron beam a rod composed of material to be evaporated, the leading end of the rod being the beam target, and means for replenishing the rod material at a rate corresponding to the rate of evaporation of the material comprising:

a. a photosensitive device responsive to radiation from the rod melt at the top of the crucible, and adapted to generate a signal proportional to the level of said melt,

. a fixed mask located opposite the rod melt and having an aperture that is variable in width for passing radiation of varying intensity according to the height of the melt meniscus,

. a transparent shielding film disposed for continuous movement between the melt and photosensitive device for precluding progressive accumulation of evaporant in the path of sensed radiation,

. means for focusing the variable intensity radiation on the photosensitive device,

. a control amplifier responsive to said signal for producing a control voltage of proportionate magnitude and a servo mechanism having a linear voltage-speed characteristic responsive to the control voltage for advancing the rod into the crucible at a rate maintaining the melt meniscus at the constant level for a constant rate of evaporation.

* It t t 

1. In an electron beam evaporation system having an evaporation chamber, an electron beam gun and a consumable target composed of material to be evaporated from a melt within the chamber, the method of maintaining an established equilibrium evaporation rate of the mateRial which comprises: a. sensing the intensity of light radiation from the material at the meniscus level of the melt; b. varying the intensity of the sensed radiation in accordance with changes in the meniscus level thereof; and c. supplying the material to the melt at a rate according to the intensity of the sensed radiation for maintaining the meniscus level at the constant equilibrium level.
 2. The method as specified in claim 1 wherein the melt at the meniscus is a source of light radiation, and the radiated light for sensing is masked and restricted to a path of graduated cross-sectional area which varies in size with variations in the meniscus level.
 3. In an electron beam evaporation system having an evaporation chamber, an electron beam gun for generating an electron beam, a consumable target for the beam composed of material to be melted to form a melt and then evaporated within the chamber, and drive means for replenishing the vaporized target material so as to maintain a constant equilibrium evaporation rate, the improvement comprising a. sensing means responsive to radiation from the target melt and for producing signal corresponding to the magnitude of such radiation, b. means for varying the intensity of radiation received at the sensing means according to the height of the melt meniscus, c. drive control means connected to the said signal and responsive thereto to control the rate of replenishing of the vaporized material at a rate for maintaining the meniscus at the constant level.
 4. An electron beam evaporation system as specified in claim 3 wherein the material is in the form of a rod and the advancing end of the rod constitutes the target for the electron beam.
 5. An electron beam evaporation system as specified in claim 3 wherein the sensing means includes photosensitive means for producing a control signal for the drive means.
 6. An electron beam evaporation system as specified in claim 5 wherein the drive means comprises an electric servo mechanism that is responsive to the control signal for advancing the rod toward the electron beam.
 7. An electron beam evaporation system as specified in claim 3 wherein the means for varying the intensity of radiation comprises a fixed mask having an aperture of graduated cross-sectional area interposed between the melt meniscus and the sensing means, whereby the intensity of sensed radiation varies according to the meniscus height.
 8. An electron beam evaporation system as specified in claim 7 wherein the mask aperture consists of a V-shaped notch, the notch at its median width being in general horizontal alignment with the melt meniscus at the constant reference height.
 9. An electron beam evaporation system as specified in claim 3 including an expendable transparent evaporant shield disposed within the chamber in the path of the variable intensity radiation to the sensing means, and shield moving means connected thereto for continuously moving the evaporant exposed area thereof from the radiation path.
 10. An electron beam evaporation system as specified in claim 5 further including an optical system between said melt and said sensing means having means for restricting radiation and focusing the restricted radiation on the photosensitive means.
 11. An electron beam evaporation system as specified in claim 4 wherein the evaporation chamber includes a crucible having a vertical bore, and the rod is advanced by the drive means through the bore and into the crucible.
 12. An electron beam evaporation system as specified in claim 11 wherein the level of the melt meniscus extends beyond the upper surface of the crucible for passing radiation horizontally to the sensing means.
 13. An electron beam evaporation system having an evaporation chamber containing an electron beam gun, a crucible for vertically guiding into the electron beam a rod composed of material to be evaporated, the leading end of the rod being the beam target, and means for replenishing the rod material at a rate corresponding to the rate of evaporation of the material comprising: a. a photosensitive device responsive to radiation from the rod melt at the top of the crucible, and adapted to generate a signal proportional to the level of said melt, b. a fixed mask located opposite the rod melt and having an aperture that is variable in width for passing radiation of varying intensity according to the height of the melt meniscus, c. a transparent shielding film disposed for continuous movement between the melt and photosensitive device for precluding progressive accumulation of evaporant in the path of sensed radiation, d. means for focusing the variable intensity radiation on the photosensitive device, e. a control amplifier responsive to said signal for producing a control voltage of proportionate magnitude and f. a servo mechanism having a linear voltage-speed characteristic responsive to the control voltage for advancing the rod into the crucible at a rate maintaining the melt meniscus at the constant level for a constant rate of evaporation. 